Method and Apparatus for Overlay Measurement

ABSTRACT

A method of measurement of at-resolution overlay offset may be implemented in a scatterometer. At least three targets are provided on a wafer, each target comprising a first marker grating and a second interleaved marker grating and each target having a different overlay bias between its first and second marker. The first and second markers are provided by subsequent lithography steps in a double patterning lithographic process. The targets are measured with a scatterometer and for each target a measured CD of at least one of the markers is determined using reconstruction. The CD of the first marker may be fixed in the reconstruction. The measured CDs and at least one of the overlay biases is used to determine an overlay result corresponding to a minimum measured CD. The overlay result may be determined by fitting a function such as a parabola to the measured CDs and the overlay biases and determining the overlay at the minimum of the fitted function.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claim benefit under 35 U.S.C. §119(e) to U.S.Provisional Patent Application No. 61/286,541, filed Dec. 15, 2009,which is incorporated by reference herein in its entirety.

BACKGROUND Field of the Invention

The present invention relates to measurement of overlay usable, forexample, in the manufacture of devices by lithographic techniques.Specifically, the present invention relates to measuring targetscomprising a first marker and a second marker and each target having adifferent overlay bias between its first and second marker and using themeasurements to determine an overlay result.

BACKGROUND ART

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,comprising part of, one, or several dies) on a substrate (e.g., asilicon wafer). Transfer of the pattern is typically via imaging onto alayer of radiation-sensitive material (resist) provided on thesubstrate. In general, a single substrate will contain a network ofadjacent target portions that are successively patterned. Knownlithographic apparatus include so-called steppers, in which each targetportion is irradiated by exposing an entire pattern onto the targetportion at one time, and so-called scanners, in which each targetportion is irradiated by scanning the pattern through a radiation beamin a given direction (the “scanning”-direction) while synchronouslyscanning the substrate parallel or anti parallel to this direction. Itis also possible to transfer the pattern from the patterning device tothe substrate by imprinting the pattern onto the substrate.

In order to monitor the lithographic process, it is necessary to measureparameters of the patterned substrate, for example the overlay errorbetween successive layers formed in or on it. There are varioustechniques for making measurements of the microscopic structures formedin lithographic processes, including the use of scanning electronmicroscopes and various specialized tools. A fast and non-invasive formof specialized inspection tool is a scatterometer in which a beam ofradiation is directed onto a target on the surface of the substrate andproperties of the scattered or reflected beam are measured. By comparingthe properties of the beam before and after it has been reflected orscattered by the substrate, the properties of the substrate can bedetermined. This can be done, for example, by comparing the reflectedbeam with data stored in a library of known measurements associated withknown substrate properties. Two main types of scatterometer are known.Spectroscopic scatterometers direct a broadband radiation beam onto thesubstrate and measure the spectrum (intensity as a function ofwavelength) of the radiation scattered into a particular narrow angularrange. Angularly resolved scatterometers use a monochromatic radiationbeam and measure the intensity of the scattered radiation as a functionof angle.

The 32 nm half pitch (HP) node and beyond will require double patterningimmersion lithography (while Extreme Ultra Violet lithography is notready for mass production). Various known process schemes exist forachieving 32 nm HP, amongst them are Litho Freeze Litho Etch (LFLE) andLitho Etch Litho Etch (LELE).

In order to establish acceptable overlay control for control between thetwo layers, overlay can be measured in various ways. However, overlay istypically not measured at the printed resolution because its measurementis not straightforward. Usually overlay is measured on specificallydesigned macro-pitch targets and measured using image based orscatterometry based methods. These special features usually do notresemble the product or product pitch. Therefore the link with the realat-resolution overlay is lost. Furthermore, scatterometry methods cannotreconstruct the dual line pattern of double patterning structures atresolution due to loss of sensitivity near the symmetry point.

Overlay at-resolution is sometimes measured on resolution targets usinga CD (Critical Dimensions) SEM (Scanning Electron Microscope). Thatmethod is expensive, takes time and is not accurate enough (estimated at˜1-2 nm).

SUMMARY

It is desirable to provide a system that accurately measuresat-resolution overlay.

According to a first aspect of the present invention, there is provideda method of measurement of overlay offset on a substrate comprising atleast three targets. Each target comprises first and second markers.Each target has a different respective predetermined overlay biasbetween its first and second markers. The method comprises the followingsteps, no necessarily in order. Measuring the targets. Determining foreach target a measured dimension. Using the measured dimensions and atleast one of the predetermined overlay biases to determine an overlayoffset result corresponding to a minimum dimension.

According to a second aspect of the present invention, there is providedan inspection apparatus for measuring an overlay offset on a substratecomprising at least three targets. Each target comprises first andsecond markers. Each target has a different respective predeterminedoverlay bias between its first and second markers. The inspectionapparatus comprises a projection system configured to project aradiation beam onto each of the targets, a detector configured to detectradiation having interacted with each of the targets, and a processorconfigured to determine for each target a measured dimension using thedetected radiation and to use the measured dimensions and at least oneof the predetermined overlay biases to determine an overlay offsetresult corresponding to a minimum dimension.

According to a third aspect of the present invention, there is provideda lithographic apparatus comprising an inspection apparatus according tothe second aspect.

According to a fourth aspect of the present invention, there is provideda computer program comprising one or more sequences of machine-readableinstructions allowing an apparatus to perform a method according to thefirst aspect.

According to a fifth aspect of the present invention, there is provideda data storage medium having a computer program comprising one or moresequences of machine-readable instructions enabling an apparatus toperform a method according to the first aspect stored therein.

Further features and advantages of the invention, as well as thestructure and operation of various embodiments of the invention, aredescribed in detail below with reference to the accompanying drawings.It is noted that the invention is not limited to the specificembodiments described herein. Such embodiments are presented herein forillustrative purposes only. Additional embodiments will be apparent topersons skilled in the relevant art(s) based on the teachings containedherein.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the relevant art(s) to makeand use the invention

FIG. 1 depicts a lithographic apparatus.

FIG. 2 depicts a lithographic cell or cluster.

FIG. 3 depicts a first scatterometer.

FIG. 4 depicts a second scatterometer.

FIG. 5 illustrates a prior art Litho Freeze Litho Etch (LFLE) processfor double patterning lithography of a grating structure.

FIG. 6 illustrates a prior art Litho Etch Litho Etch (LELE) process fordouble patterning lithography of a grating structure.

FIG. 7 illustrates the effect of an overlay offset on the CD of resistbars for the LFLE process.

FIG. 8 illustrates a graph of reconstructed critical dimension CDagainst a deliberately applied mask overlay bias OVB between the twoexposure steps in the LFLE process.

FIG. 9 illustrates the results of the application of an embodiment ofthe present invention with a graph of overlay (OV) against positionacross a wafer with an imposed wafer scaling offset.

FIG. 10 illustrates steps of a method of measurement of lateral overlayoffset according to an embodiment of the present invention.

The features and advantages of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings, in which like reference charactersidentify corresponding elements throughout. In the drawings, likereference numbers generally indicate identical, functionally similar,and/or structurally similar elements. The drawing in which an elementfirst appears is indicated by the leftmost digit(s) in the correspondingreference number.

DETAILED DESCRIPTION

This specification discloses one or more embodiments that incorporatethe features of this invention. The disclosed embodiment(s) merelyexemplify the invention. The scope of the invention is not limited tothe disclosed embodiment(s). The invention is defined by the claimsappended hereto.

The embodiment(s) described, and references in the specification to “oneembodiment”, “an embodiment”, “an example embodiment”, etc., indicatethat the embodiment(s) described may include a particular feature,structure, or characteristic, but every embodiment may not necessarilyinclude the particular feature, structure, or characteristic. Moreover,such phrases are not necessarily referring to the same embodiment.Further, when a particular feature, structure, or characteristic isdescribed in connection with an embodiment, it is understood that it iswithin the knowledge of one skilled in the art to effect such feature,structure, or characteristic in connection with other embodimentswhether or not explicitly described.

Embodiments of the invention may be implemented in hardware, firmware,software, or any combination thereof. Embodiments of the invention mayalso be implemented as instructions stored on a machine-readable medium,which may be read and executed by one or more processors. Amachine-readable medium may include any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputing device). For example, a machine-readable medium may includeread only memory (ROM); random access memory (RAM); magnetic diskstorage media; optical storage media; flash memory devices; electrical,optical, acoustical or other forms of propagated signals (e.g., carrierwaves, infrared signals, digital signals, etc.), and others. Further,firmware, software, routines, instructions may be described herein asperforming certain actions. However, it should be appreciated that suchdescriptions are merely for convenience and that such actions in factresult from computing devices, processors, controllers, or other devicesexecuting the firmware, software, routines, instructions, etc.

Before describing such embodiments in more detail, however, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

FIG. 1 schematically depicts a lithographic apparatus. The apparatuscomprises an illumination system (illuminator) IL configured tocondition a radiation beam B (e.g., UV radiation or DUV radiation), asupport structure (e.g., a mask table) MT constructed to support apatterning device (e.g., a mask) MA and connected to a first positionerPM configured to accurately position the patterning device in accordancewith certain parameters, a substrate table (e.g., a wafer table) WTconstructed to hold a substrate (e.g., a resist coated wafer) W andconnected to a second positioner PW configured to accurately positionthe substrate in accordance with certain parameters, and a projectionsystem (e.g., a refractive projection lens system) PL configured toproject a pattern imparted to the radiation beam B by patterning deviceMA onto a target portion C (e.g., comprising one or more dies) of thesubstrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e., bears the weight of, thepatterning device. It holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.,employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g., employing a programmable mirror array of a typeas referred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the mask MA, theradiation beam B passes through the projection system PL, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF (e.g., an interferometricdevice, linear encoder, 2-D encoder or capacitive sensor), the substratetable WT can be moved accurately, e.g., so as to position differenttarget portions C in the path of the radiation beam B. Similarly, thefirst positioner PM and another position sensor (which is not explicitlydepicted in FIG. 1) can be used to accurately position the mask MA withrespect to the path of the radiation beam B, e.g., after mechanicalretrieval from a mask library, or during a scan. In general, movement ofthe mask table MT may be realized with the aid of a long-stroke module(coarse positioning) and a short-stroke module (fine positioning), whichform part of the first positioner PM. Similarly, movement of thesubstrate table WT may be realized using a long-stroke module and ashort-stroke module, which form part of the second positioner PW. In thecase of a stepper (as opposed to a scanner) the mask table MT may beconnected to a short-stroke actuator only, or may be fixed. Mask MA andsubstrate W may be aligned using mask alignment marks M1, M2 andsubstrate alignment marks P1, P2. Although the substrate alignment marksas illustrated occupy dedicated target portions, they may be located inspaces between target portions (these are known as scribe-lane alignmentmarks). Similarly, in situations in which more than one die is providedon the mask MA, the mask alignment marks may be located between thedies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e., asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e., a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

As shown in FIG. 2, the lithographic apparatus LA forms part of alithographic cell LC, also sometimes referred to a lithocell or cluster,which also includes apparatus to perform pre- and post-exposureprocesses on a substrate. Conventionally these include spin coaters SCto deposit resist layers, developers DE to develop exposed resist, chillplates CH and bake plates BK. A substrate handler, or robot, RO picks upsubstrates from input/output ports I/O1, I/O2, moves them between thedifferent process apparatus and delivers then to the loading bay LB ofthe lithographic apparatus. These devices, which are often collectivelyreferred to as the track, are under the control of a track control unitTCU which is itself controlled by the supervisory control system SCS,which also controls the lithographic apparatus via lithography controlunit LACU. Thus, the different apparatus can be operated to maximizethroughput and processing efficiency.

In order that the substrates that are exposed by the lithographicapparatus are exposed correctly and consistently, it is desirable toinspect exposed substrates to measure properties such as overlay errorsbetween subsequent layers, line thicknesses, critical dimensions (CD),etc. If errors are detected, adjustments may be made to exposures ofsubsequent substrates, especially if the inspection can be done soon andfast enough that other substrates of the same batch are still to beexposed. Also, already exposed substrates may be stripped andreworked—to improve yield—or discarded, thereby avoiding performingexposures on substrates that are known to be faulty. In a case whereonly some target portions of a substrate are faulty, further exposurescan be performed only on those target portions which are good.

An inspection apparatus is used to determine the properties of thesubstrates, and in particular, how the properties of differentsubstrates or different layers of the same substrate vary from layer tolayer. The inspection apparatus may be integrated into the lithographicapparatus LA or the lithocell LC or may be a stand-alone device. Toenable most rapid measurements, it is desirable that the inspectionapparatus measure properties in the exposed resist layer immediatelyafter the exposure. However, the latent image in the resist has a verylow contrast—there is only a very small difference in refractive indexbetween the parts of the resist which have been exposed to radiation andthose which have not—and not all inspection apparatus have sufficientsensitivity to make useful measurements of the latent image. Thereforemeasurements may be taken after the post-exposure bake step (PEB) whichis customarily the first step carried out on exposed substrates andincreases the contrast between exposed and unexposed parts of theresist. At this stage, the image in the resist may be referred to assemi-latent. It is also possible to make measurements of the developedresist image—at which point either the exposed or unexposed parts of theresist have been removed—or after a pattern transfer step such asetching. The latter possibility limits the possibilities for rework offaulty substrates but may still provide useful information.

FIG. 3 depicts a scatterometer which may be used in the presentinvention. It comprises a broadband (white light) radiation projector 2which projects radiation onto a substrate W. The reflected radiation ispassed to a spectrometer detector 4, which measures a spectrum 10(intensity as a function of wavelength) of the specular reflectedradiation. From this data, the structure or profile giving rise to thedetected spectrum may be reconstructed by processing unit PU, e.g., byRigorous Coupled Wave Analysis and non-linear regression or bycomparison with a library of simulated spectra as shown at the bottom ofFIG. 3. In general, for the reconstruction the general form of thestructure is known and some parameters are assumed from knowledge of theprocess by which the structure was made, leaving only a few parametersof the structure to be determined from the scatterometry data. Such ascatterometer may be configured as a normal-incidence scatterometer oran oblique-incidence scatterometer.

Another scatterometer that may be used with the present invention isshown in FIG. 4. In this device, the radiation emitted by radiationsource 2 is collimated using lens system 12 and transmitted throughinterference filter 13 and polarizer 17, reflected by partiallyreflected surface 16 and is focused onto substrate W via a microscopeobjective lens 15, which has a high numerical aperture (NA), preferablyat least 0.9 and more preferably at least 0.95. Immersion scatterometersmay even have lenses with numerical apertures over 1. The reflectedradiation then transmits through partially reflective surface 16 into adetector 18 in order to have the scatter spectrum detected. The detectormay be located in the back-projected pupil plane 11, which is at thefocal length of the lens system 15, however the pupil plane may insteadbe re-imaged with auxiliary optics (not shown) onto the detector. Thepupil plane is the plane in which the radial position of radiationdefines the angle of incidence and the angular position defines azimuthangle of the radiation. The detector is preferably a two-dimensionaldetector so that a two-dimensional angular scatter spectrum of asubstrate target 30 can be measured. The detector 18 may be, forexample, an array of CCD or CMOS sensors, and may use an integrationtime of, for example, 40 milliseconds per frame.

A reference beam is often used for example to measure the intensity ofthe incident radiation. To do this, when the radiation beam is incidenton the beam splitter 16 part of it is transmitted through the beamsplitter as a reference beam towards a reference minor 14. The referencebeam is then projected onto a different part of the same detector 18.

A set of interference filters 13 is available to select a wavelength ofinterest in the range of, say, 405-790 nm or even lower, such as 200-300nm. The interference filter may be tunable rather than comprising a setof different filters. A grating could be used instead of interferencefilters.

The detector 18 may measure the intensity of scattered light at a singlewavelength (or narrow wavelength range), the intensity separately atmultiple wavelengths or integrated over a wavelength range. Furthermore,the detector may separately measure the intensity of transversemagnetic- and transverse electric-polarized light and/or the phasedifference between the transverse magnetic- and transverseelectric-polarized light.

Using a broadband light source (i.e., one with a wide range of lightfrequencies or wavelengths—and therefore of colors) is possible, whichgives a large etendue, allowing the mixing of multiple wavelengths. Theplurality of wavelengths in the broadband preferably each has abandwidth of Δλ and a spacing of at least 2Δλ (i.e., twice thebandwidth). Several “sources” of radiation can be different portions ofan extended radiation source which have been split using fiber bundles.In this way, angle resolved scatter spectra can be measured at multiplewavelengths in parallel. A 3-D spectrum (wavelength and two differentangles) can be measured, which contains more information than a 2-Dspectrum. This allows more information to be measured which increasesmetrology process robustness. This is described in more detail in EPApplication No. 1,628,164A, which is incorporated by reference herein inits entirety.

The target 30 on substrate W may be a 1-D grating, which is printed suchthat after development, the bars are formed of solid resist lines. Thetarget 30 may be a 2-D grating, which is printed such that afterdevelopment, the grating is formed of solid resist pillars or vias inthe resist. The bars, pillars or vias may alternatively be etched intothe substrate. This pattern is sensitive to chromatic aberrations in thelithographic projection apparatus, particularly the projection systemPL, and illumination symmetry and the presence of such aberrations willmanifest themselves in a variation in the printed grating. Accordingly,the scatterometry data of the printed gratings is used to reconstructthe gratings. The parameters of the 1-D grating, such as line widths andshapes, or parameters of the 2-D grating, such as pillar or via widthsor lengths or shapes, may be input to the reconstruction process,performed by processing unit PU, from knowledge of the printing stepand/or other scatterometry processes.

FIG. 5 illustrates the prior art Litho Freeze Litho Etch (LFLE) processfor double patterning lithography of a grating structure.

Step 502 is the first exposure and development step that results in agrating pattern with first resist bars 514 shown in cross section on thesubstrate 516. The next step 504 is coating the substrate with across-linking material 518. A mixing bake 506 is performed that resultsin a cross-linked layer 520 at the interface of the resist bars with thecross-linking material. A development step 508 removes thenon-cross-linked residues. A resist coating step 510 results in a secondlayer of resist 522. The second layer of resist 522 is patterned in asecond exposure and development step 512 that results in an interleavedgrating structure with second resist bars 524 in between the resist bars514 formed in the first exposure and development step 502.

In double patterning lithography it is desirable to obtain accurateoverlay of the exposure steps 502 and 512 so that there is uniformspacing between the respective pattern features, in this simple case ofa grating that is the resist bars 514 and 524.

FIG. 6 illustrates the prior art Litho Etch Litho Etch (LELE) processfor double patterning lithography of a grating structure. This is a“positive” lithography LELE process that produces a hard mask grating.In the first exposure and development step 602 a reticle 610 having apattern 612 is used to create a corresponding resist structure 614 onthe Bottom Anti-Reflective Coating (BARC) layer 616. The underlyinglayers comprises a hard mask layer 618 on polysilicon 620 with anunderlying layer of silicon dioxide 622. An etch step 604 transfers thepattern from the resist structure 614 into the hard mask layer 618 toproduce a hard mask pattern 624. After a deposition coating steps (notshown), a second exposure step 606 uses a second reticle 626 having apattern 628 to expose a coated resist layer 630 on deposited hard masklayer 632. After developing and etching, the result in step 608 is astructure with features 624 and 634 interleaved to provide a hard maskgrating structure, which may then be used as an etch mask to transferthe pattern into the underlying layer.

As with LFLE, in LELE, it is desirable to have accurate overlay betweenthe two exposure steps 602 and 606 in order to provide uniform spacingbetween features produced on the substrate shown in step 608.

When a reticle (e.g., 626) for the second exposure step (e.g., in steps512, 606) contains multiple targets with predetermined imposed maskoverlay bias in the second printed bar with respect to the first printedbar, the reconstructed CD behaves systematically with the imposedoffset. FIG. 7 illustrates the effect of an overlay offset OV on the CDof resist bars for the LFLE process. In FIG. 7, two cross sections 702and 704 are shown of grating targets produced by the LFLE process, forexample as described with reference to FIG. 5.

The first cross section 702 shows a grating target made of resist bars706 (corresponding to 514 in FIG. 5) with interleaved resist bars 708(corresponding to 524 in FIG. 5). The substrate is represented by theline 710. In cross section 702 the overlay offset is zero and there iseven spacing on either side of resist bars 708 with respect to resistbars 706. Furthermore, the exposure of the resist bars 708 at the secondexposure and development step has been controlled to give a criticaldimension CD1 that is equal to the critical dimension of the resist bars706 created by the first exposure and development step.

The second cross section 704 illustrates the interdependence of overlayand critical dimension in the LFLE process. The second bars 712 areshifted to the right compared to cross section 702 by an overlay offsetOV. It is observed that this causes the critical dimension of the secondprinted bars 712 to increase to CD2, which is larger compared to thedimension CD1 of bar 708 in cross section 702.

There are various mechanisms that may result in CD being dependent onoverlay offset in such structures. For example, exposure of the resist(522 in FIG. 5) in proximity to the first bars 514 may be affected byscattering of exposure radiation by the structures 514 and 520 or byresist 522 thickness variation in close proximity to the first bars 514.

Although resist bars have been shown in these examples, these effectsand the application of the present invention are not limited to 1-Dgratings made of bars. 2-D gratings may be used. Other structures moreclosely resembling semiconductor device structures may also be used tomimic and allow closer control of the semiconductor device fabricationprocess. Overlay is usually a 2-D X-Y metric. Although this embodimentrelates to overlay in one direction, the present invention may beapplied the orthogonal direction in addition. If the target is 2-Dperiodic, then still a minimum three targets are needed to determine theoverlay in by fitting in two orthogonal directions.

FIG. 8 illustrates a graph 802 of reconstructed critical dimension CDmeasured in nanometers (nm) against a deliberately applied mask overlaybias OVB (in nm) between the two exposure steps in the LFLE process.FIG. 8 shows results for nested CD targets each of nominal 32 nmlinewidth. The first exposure step overlay bias is zero for all datapoints and the second exposure step overlay biases are −10, −8, −6, 0,+6, +8 and +10 nm respectively. Alternatively, the biases may be appliedin the first exposure step, or a combination of the first or secondexposure steps.

In this example, regression analysis is used to fit a parabola 804 tothe measured data points 806. Although a parabola is used in thisexample, other functions could be used, so long as they allow theidentification of a minimum CD value that corresponds to the overlayoffset OV. Instead of fitting a function, the minimum CD value could beobtained directly from the measurements by finding the lowest measuredCD and using its corresponding overlay bias value as the estimate of theoverlay offset. However, function fitting requires fewer measurements toobtain more accurate results. In this example, fitting the parabola anddetermining the minimum yields the overlay. At minimum three targets areneeded to determine the overlay by fitting; measuring more targets willgive a better representation of the minimum. At larger printed overlayoffset values it is found that the curve is a sine-like curve.

The CD of the interleaved at-resolution bars (708 and 712 in FIG. 7) maybe determined using known reconstruction methods. In the reconstructionmodel fixed parameters are used for the profile for the first bars(features 706 in FIG. 7). The reconstruction method may then yield amodeled value for the CD of the second bars at each overlay bias, i.e.,data points 806. Thus, a scatterometry system can measure the relativeline width of the second bar by fixing the first bar in thescatterometry system's reconstruction model. The overlay at which themodeled CD value is a minimum is then obtained as the estimator for theoverlay. However, fixing the first bar profile to the nominal settingsdoes not change the overlay measurement results. The only resultingdifference from taking different starting parameters for profile of thefirst bar is the lifting of the constant in the equation of the fittedparabola. This means that the determined CD of the second bar does nothave accurate representation of the line width itself, but it is still agood means to determine overlay.

In this example, the minimum of the parabola 804 determined by theregression is used as an estimator for the overlay. In this case aresult of overlay of 0.89 nanometres is obtained. In experiments, therepeatability over ten runs has been found to be 0.2 nanometres.

The required mask bias is determined depending on the overlay valuesthat are expected. In this example a range between +/−10 nm was used foroverlay values in the same range. This makes this technique verysuitable for small range overlay metrology.

FIG. 9 illustrates the results of the application of an embodiment ofthe present invention. The graph 902 shows overlay (OV) measured innanometres against an X position (measured in millimeters) across awafer with an imposed wafer scaling offset. Linear regression is used tofit the line 904 to the measured data points. Repeating the measurementten times shows a repeatability of 0.2 nm.

FIG. 10 illustrates the steps of a method of measurement of lateraloverlay offset according to an embodiment of the present invention.

Step 1002 is providing at least three targets, each target comprising afirst marker (grating) and a second marker (interleaved grating) andeach target having a different overlay bias between its first and secondmarker. The first and second markers are provided by subsequentlithography steps in a double patterning lithographic process such asLFLE or LELE.

Step 1004 is measuring the targets and step 1006 is determining for eachtarget a measured CD of at least one of the markers usingreconstruction. The CD of the first marker may be fixed in thereconstruction.

Step 1008 is using the measured CDs and at least one of the overlaybiases to determine an overlay result corresponding to a minimummeasured CD. The overlay result may be determined by fitting a functionto the measured CDs and the overlay biases and determining the overlayat the minimum of the fitted function. Alternatively, the overlay resultis determined by finding the smallest of the measured CDs and using itscorresponding overlay bias to determine the overlay result.

In the manufacture of lithographic apparatus, the present invention issuitable for the qualification of scanners for LFLE double patterning,instead of using slow, costly and less accurate SEM measurements. LFLEis preferable for such qualification applications because there is noneed for etch equipment in the factory. In production of semiconductordevices, LELE is more useful. In LELE, there is a coupling betweenoverlay and CD, therefore the present invention is applicable to theLELE process. Furthermore, the present invention is applicable to anyprocess where there is a coupling between overlay and CD and it ispossible to determine a minimum CD.

Although specific reference may be made in this text to the use ofinspection apparatus in the manufacture of ICs, it should be understoodthat the inspection apparatus described herein may have otherapplications, such as the manufacture of integrated optical systems,guidance and detection patterns for magnetic domain memories, flat-paneldisplays, liquid-crystal displays (LCDs), thin film magnetic heads, etc.The skilled artisan will appreciate that, in the context of suchalternative applications, any use of the terms “wafer” or “die” hereinmay be considered as synonymous with the more general terms “substrate”or “target portion”, respectively. The substrate referred to herein maybe processed, before or after exposure, in for example a track (a toolthat typically applies a layer of resist to a substrate and develops theexposed resist), a metrology tool and/or an inspection tool. Whereapplicable, the disclosure herein may be applied to such and othersubstrate processing tools. Further, the substrate may be processed morethan once, for example in order to create a multi-layer IC, so that theterm substrate used herein may also refer to a substrate that alreadycontains multiple processed layers.

A stand-alone computer or the processing unit PU described above withreference to FIGS. 3 and 4 may be configured to perform the steps ofdetermining CD for each target and determining an overlay offset resultas described with reference to steps 1006 and 1008 in FIG. 10.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g., having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g., semiconductor memory, magnetic or optical disk) havingsuch a computer program stored therein.

It is to be appreciated that the Detailed Description section, and notthe Summary and Abstract sections, is intended to be used to interpretthe claims. The Summary and Abstract sections may set forth one or morebut not all exemplary embodiments of the present invention ascontemplated by the inventor(s), and thus, are not intended to limit thepresent invention and the appended claims in any way.

The present invention has been described above with the aid offunctional building blocks illustrating the implementation of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the teachings andguidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

The claims in the instant application are different than those of theparent application or other related applications. The Applicanttherefore rescinds any disclaimer of claim scope made in the parentapplication or any predecessor application in relation to the instantapplication. The Examiner is therefore advised that any such previousdisclaimer and the cited references that it was made to avoid, may needto be revisited. Further, the Examiner is also reminded that anydisclaimer made in the instant application should not be read into oragainst the parent application.

1. A method comprising: measuring first through third targets havingfirst and second markers and different respective predetermined overlaybias between respective ones of the first and second markers;determining a measured dimension for each of the first through thirdtargets; and using the measured dimensions and at least one of thepredetermined overlay biases to determine an overlay offset resultcorresponding to a minimum dimension.
 2. The method according to claim1, wherein: the measuring comprises, projecting a radiation beam ontoeach of the first through third targets; detecting radiation havinginteracted with each of the first through third targets; and wherein thedetermining comprises using the detected radiation.
 3. The methodaccording to claim 1, wherein the measured dimension comprises ameasured dimension of at least one of the first and second markers ofthe first through third targets.
 4. The method according to claim 1,wherein the first and second markers are provided by subsequentlithography steps in a double patterning lithographic process.
 5. Themethod according to claim 1, wherein the measured dimensions aredetermined using reconstruction.
 6. The method according to claim 5,wherein a modeled dimension of one of the first and second markerscorresponding to the measured dimension of another of the first andsecond markers is fixed in the reconstruction.
 7. The method accordingto claim 1, wherein the overlay offset result is determined by fitting afunction to the measured dimensions and the predetermined overlay biasesand determining an overlay value at a minimum of the fitted function. 8.The method according to claim 1, wherein the overlay offset result isdetermined by determining a smallest of the measured dimensions andusing its corresponding predetermined overlay bias to determine theoverlay offset result.
 9. An inspection apparatus for measuring anoverlay offset on a substrate comprising three targets, each of thethree targets comprising first and second markers and each of the threetargets having a different respective predetermined overlay bias betweenrespective ones of the first and second markers, the inspectionapparatus comprising: a projection system configured to project aradiation beam onto each of the three targets; a detector configured todetect radiation having interacted with each of the three targets; and aprocessor configured to determine for each of the three targets ameasured dimension using the detected radiation and to use the measureddimensions and at least one of the predetermined overlay biases todetermine an overlay offset result corresponding to a minimum dimension.10. The inspection apparatus according to claim 9, wherein the measureddimension comprises a measured dimension of at least one of the firstand second markers of each of the three targets.
 11. The inspectionapparatus according to claim 9, wherein the first and second markers areprovided by subsequent lithography steps in a double patterninglithographic process.
 12. The inspection apparatus according to claim 9,wherein the processor is configured to determine the measured dimensionsusing reconstruction.
 13. The inspection apparatus according to claim12, wherein the processor is configured to fix in the construction amodeled dimension of one of the first and second markers correspondingto the measured dimension of another one of the first and secondmarkers.
 14. The inspection apparatus according to claim 9, wherein theprocessor is configured to determine the overlay offset result byfitting a function to the measured dimensions and the predeterminedoverlay biases and to determine an overlay value at a minimum of thefitted function.
 15. The inspection apparatus according to claim 9,wherein the processor is configured to determine a smallest of themeasured dimensions and use its corresponding predetermined overlay biasto determine the overlay offset result.
 16. A lithographic apparatuscomprising: a support configured to support a patterning deviceconfigured to pattern a beam; a projection system configured to projectthe patterned beam onto a substrate; and an inspection apparatus formeasuring an overlay offset on a substrate comprising three targets,each of the three targets comprising first and second markers and eachof the three targets having a different respective predetermined overlaybias between respective ones of the first and second markers, theinspection apparatus comprising: another projection system configured toproject a radiation beam onto each of the three targets; a detectorconfigured to detect radiation having interacted with each of the threetargets; and a processor configured to determine for each of the threetargets a measured dimension using the detected radiation and to use themeasured dimensions and at least one of the predetermined overlay biasesto determine an overlay offset result corresponding to a minimumdimension.
 17. An article of manufacture including a computer-readablemedium having instructions stored thereon, executed of which by acomputing device cause the computing device to perform operationscomprising: measuring first through third targets having first andsecond markers and different respective predetermined overlay biasbetween respective ones of the first and second markers; determining ameasured dimension for each of the first through third targets; andusing the measured dimensions and at least one of the predeterminedoverlay biases to determine an overlay offset result corresponding to aminimum dimension.
 18. A computer-readable non-transitory medium havinginstructions stored thereon, the instructions comprising: instructionsfor measuring first through third targets having first and secondmarkers and different respective predetermined overlay bias betweenrespective ones of the first and second markers; instructions fordetermining a measured dimension for each of the first through thirdtargets; and instructions for using the measured dimensions and at leastone of the predetermined overlay biases to determine an overlay offsetresult corresponding to a minimum dimension.